1. Field of the Invention
The present invention relates to gas-insulated electric apparatus and more particularly to gas-insulated electric apparatus in which a partial discharge detecting device and method are provided at an insulating supporter which supports an electric conductor carrying a high-voltage current (hereinafter simply referred to as "conductor") within an insulating-gas-filled metallic container of the apparatus such that possible partial discharges can be detected with ease.
2. Description of the Prior Art
Known gas-insulated electric apparatus, such as gas-insulated switchgear, gas-insulated buses (hermetically sealed bus) and the like, have a conductor encased in a metallic container. The conductor is supported by insulating supporter means such as an insulating spacer. In the thus arranged apparatus, it is impossible to visually inspect an accidental partial discharge along the insulating supporter. Accordingly, it is necessary to detect such partial discharges occurring in the electric apparatus of this type, exteriorly thereof with high accuracy.
The insulating supporter means herein referred to typically includes a disc-like insulating spacer for dividing the interior of the metallic container into several gas chambers as well as for supporting a conductor, a rod-like insulator for merely supporting a conductor, and the like.
The following description will be given, by way of example, on a hermetically sealed bus.
In a hermetically sealed bus, a power feed conductor or bus conductor is supported within a cylindrical metallic container by insulating spacers moulded out of an epoxy resin and a high insulation gas is filled in the container to ensure insulation between the bus conductor and the container. On the surface of each insulating spacer having the above-mentioned construction, foreign materials such as dust may be deposited to cause a partial discharge along the surface of the spacer and in the event such a partial discharge is allowed to continue for a long period under the application of a high-voltage, a tree-like discharge corrosion may be created and grown on the surface of the spacer, resulting in a serious accident.
Measures for detecting such a partial discharge have hitherto been discussed. In one of the measures, for example, it is proposed to employ a shield ring for mitigating the electric field as an electrode for the discharge detection. The shield ring, however, is connected to an embedded metallic thimble by way of metallic wires fixed during molding. This makes it difficult to float and lead out a potential at the shield ring. It is of course possible to provide a separate or independent metallic lead-out member supported by an additional spacer. But this measure requires an extensive change in the conventional fabrication process and presents a safety hazard of a high potential appearing at the lead terminal, and thus has not yet been put into practice.
As an alternative, a measure has been proposed wherein an auxiliary electrode is provided around the exposed outer peripheral surface of the insulating spacer to provide a capacity division voltage from which the discharge is detected. This latter measure as well as the former measure utilizing the shield ring, however, are defective because of their detection sensitivity.
More particularly, both of the measures discussed above have a high input impedance and give the measuring system a large magnitude of inductive noise, resulting in difficulties with the discrimination of the noise from the detection signal because such discrimination cannot be solved by merely increasing the amplification degree of the measuring instrument, thus leading to degraded detection sensitivity.
According to experiments conducted by the inventors of the present applicaton, the conventionally proposed measures were rather successful in detecting discharges of 100 to several 100 PC, but the detection level would be decreased if measured in a noisy environment such as in field test.
On the other hand, it is demanded that attention be paid to the fact that even a discharge of the order of 10 PC will degrade the insulating support, such as a spacer, which is loaded with a high electric field, and hence in a hermetically sealed bus a highly sensitive detection for inspecting partial discharges poses an important problem.
U.S. Pat. Nos. 3,652,778, 3,781,454 and 3,949,157, for example, disclose an insulating supporter which supports a conductor passing through the interior of a metallic container. Laid-open Japanese patent applications Nos. 122,786/'76, 122, 787/'76, 122, 788/'76 and 70,183/'74, for example, disclose diagnosis for the insulation of an insulating supporter.